1. Field of the Invention
This invention relates to an electronic parts-mounting apparatus including a rotatable table which carries mounting heads and intermittently rotates while enabling each mounting head to pick up an electronic part by sucking, carry the sucked electronic part, and mount the same on a circuit board, and lifting/lowering devices provided for the mounting heads for lifting and lowering the mounting heads to thereby enable each mounting head to suck or mount the electronic part during stoppage of the rotatable table, and more particularly to an improvement of a drive system of the apparatus.
2. Prior Art
Conventionally, an electronic parts-mounting apparatus of this kind includes an index unit which has its input side connected to a drive motor as a driving force source via a worm reduction gear, and its output side connected to a rotatable table carrying mounting heads, and an output shaft connected to two or three camshafts for transmitting the driving force or torque to various devices each provided for operation in synchronism with the intermittent rotation of the rotatable table. The uniform rotation of the drive motor is reduced in speed by the worm reduction gear and the resulting rotational motion is input to the index unit. The index unit, on one hand, converts the uniform rotation into intermittent rotation (index rotation) to cause the rotatable table to rotate intermittently, and on the other hand, transmits the uniform rotation (or torque) via the output shaft and a timing belt to the two or three camshafts to cause the same to rotate at the uniform rotational speed. As the rotatable table intermittently rotates, the mounting heads carried thereby rotate and stop at a sucking position, a mounting position, a correcting position, and other predetermined positions. On the other hand, as the camshafts rotate at the uniform rotational speed, various cams mounted thereon operate to lower and lift the mounting heads for sucking or mounting of electronic parts, selection of a sucking nozzle of each mounting head, and other operations, in a manner synchronous with stoppage of the rotatable table.
More specifically, the index unit causes two repetition periods of intermittent rotation (one repetition period consisting of a period of one intermittent rotational motion and a period of stoppage) of the rotatable table and at the same time one complete rotation of the output shaft at the uniform speed, per complete uniform rotation input to the index unit via the worm reduction gear. To this end, in the conventional electronic parts-mounting apparatus, a pulley mounted on the output shaft has a radius twice as large as the radius of a pulley mounted on each camshaft, whereby the camshaft is rotated at a rotational speed twice as high as the rotational speed of the output shaft to make the repetition period (intermittent period) of intermittent rotation of the rotatable table synchronous with the repetition period (operating period) of operation of the cam follower which is in rolling contact with the cam. That is, when the rotatable table is in stoppage, the lifting/lowering devices and other devices provided for the mounting heads and linked to the cam followers are each driven for performing one complete operation. Thus, the rotational speed of the rotatable table is made twice as high as that of the rotation input by the worm reduction gear, and at the same time the rotational speed of the camshafts is made twice as high as the same. This provides a measure or means for increasing the speed of the overall operation of the electronic parts-mounting apparatus using the worm reduction gear which sets a limit to the allowable input rotational speed, i.e. a measure or means for increasing the speed of mounting of electronic parts.
As described above, to make the intermittent period of the rotatable table synchronous with the operating period of the cam followers in rolling contact with the cams which are mounted on the camshafts and associated with the cam followers, respectively, the rotational speed of the camshafts is made twice as high as the rotational speed of the output shaft by means of pulleys mounted on the output shaft and the camshafts. Accordingly, the rotational speed of the cams is doubled, which causes wear of the cams and markedly increases thrusting forces applied to the cam followers, resulting in shortened lives of the cam followers and bearings of the camshafts.